[argyllcms] Re: Color errors on < 100% stimulus

  • From: Kristian Jörg <krjg@xxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Mon, 02 Apr 2012 15:43:09 +0200

János, Tóth F. skrev 2012-03-26 00:56:

I still don't understand the so-called „color decoding issue”.
My Pana G30E has relatively good and significantly better color
accuracy than my Samsung D550 had.
According to avsforum threads there is an issue with at least the American versions of the GT and VT models. The issues are visible with a video sequence with a woman in purle dress that was decoded as blue - on a otherwise perfectly calibrated set. There has also been reports on lightness nonlinearities over different intensities/saturations - this is supposedly fixed by named patch.
The problems I have myself is that if I calibrate my set to perfection with the tools and methods as usual, I end up with a set that measures perfectly with dE < 1.0 over the scale - but visibly has color errors with normal content! I just now calibrated my PC monitor (at last) and it shows no color casts whatever. Perfect calibration/profiling with Argyll. So it is not my meter that is malfunctioning... For the moment I have reverted to the stock THX mode which is fairly accurate. I'll try again to calibrate some day.
How do you know that the 2012 models will not be improved with regard to CMS? They have hardly come out yet...
Chromapure has options for calibrationg at 75% saturated patterns. These can be found on the AVSHD 709 disc under the HCFR header and the "Saturation" patterns. Use a 100% stimulus white pattern and then the 75% saturated.
Actually, I haven't found any instructions on how to use the 75% intenity color patterns or when they are appropriate? 

/Kristian

2012 won't be very different except the VT50.

75% saturated pattern?
I never heard about such thing. On the other hand, 75% stimulus is
very common for HDTV calibration.

2012/3/21, Kristian Jörg<krjg@xxxxxxx>:

Graeme Gill skrev 2012-02-13 14:41:

Kristian Jörg wrote:

Well, the reason I was a bit unimpressed is because this is supposed to
be one of the very best TV
sets to buy, and it includes full ISF controls for managing grayscale at
10% stimulus steps. Color
however is only at 100% stimulus. So I can nail in the grayscale and
gamma to mostly perfect result
(according to the patterns used, i.e full windows cause lightness to drop
a bit). Color however
seems a bit more difficult. Now it is possible I have used the CP
software wrongly at the 75% color
intensity. It has a 75% Rec. 709 setting that I assumed should be used
for the 75% W+ 75% RGB
measurements. But maybe I should measure all those at 100% Rec. 709
setting since i assume the Rec
709 standard is linear? When using 75% patterns all that happens is that
luminocity falls a bit,
but it is all realite to white which is measured as a baseline... ?

I'm afraid I have to smile a bit at all that. Rec 709 is an idealised
additive display.
Idealised displays have a perfect black, something no real display can
achieve.
So (as usual) you are faced with the issue of how do you deal with the
non-perfect black,
given that the transfer curve assumes that is is perfect. Non-LCD based
displays are not
so bad in this regard though. Then there are the viewing condition
assumptions inherent
in Rec 709.

The channel combination targets aren't too hard to compute - add the XYZ
of the individual channel
target or measurement values, since Rec 709 is additive.

Graeme Gill.

Just a short follow up to this little thread that I started. It was all
due to a misunderstanding of what patterns to use for the 75% Rec 709
calibration settings in Chromapure. It should be 75% saturated, not 75%
intensity. With correct patterns my results where correct. However if
anybody tries this, there is a confirmed bug with it with at least
version 2.0.10 (and probably 2.0.11) in CP, so wait for the next release
before trying this.

And there is also strong indication of color decoding issues in the 2011
models of Panasonic plasmas. Something that should be fixed in the 2012
models. Bummer. There will not be a fix to the 2011 models, or even any
more firmaware updates at al,l according to those who "know".

/Kristian

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